1. Field of the Invention
The invention relates to a polarization beam splitter and a projection display apparatus in which the polarization beam splitter functions as a polarizer and an analyzer.
2. Description of the Related Art
As a technique of forming an image on a large screen, a method is well known in which an optical image corresponding to a video signal is formed on a light valve, the light valve is irradiated with light, and the optical image formed on the light valve is projected by a projection lens onto a screen while magnifying the image. Recently, a projection display apparatus in which a liquid crystal display is used as a light valve has attraction. For example, M. HIMURO proposes in Japanese Laid-Open Patent Publication No. 61-13885 a method in which the pixel number of a liquid crystal display can be increased or a high resolution can be attained without reducing the pixel aperture factor, by using a reflection-type liquid crystal display.
FIG. 16 shows an outline of a projection display apparatus using the reflection-type liquid crystal display. Light 2 which is emitted from a light source 1 so as to be substantially parallel is directed to a polarization beam splitter 3. The polarization beam splitter 3 reflects S-polarized light 4, and allows P-polarized light 5 to pass therethrough. The reflected S-polarized light 4 enters a liquid crystal display 6 in which each pixel is provided with a reflective electrode for reflecting light. When no voltage is applied to a liquid crystal layer of the liquid crystal display 6, the liquid crystal layer does not substantially show birefringence. In contrast, when a voltage is applied to the liquid crystal layer, the liquid crystal layer shows birefringence. Therefore, when linearly polarized light which is polarized in a predetermined direction enters the liquid crystal layer to which a voltage is applied, elliptically polarized light is reflected from the liquid crystal display. In other words, a portion of the S-polarized light 4 is converted into P-polarized light by the liquid crystal display 6. The reflected light including the S and P-polarized light enters again the polarization beam splitter 3. The P-polarized light included in the light reflected from the liquid crystal display 6 passes through the polarization beam splitter 3 to enter a projection lens 7, and the S-polarized light is reflected by the beam splitter to proceed toward the light source 1. In this way, an optical image produced in the form of a change of birefringence on the liquid crystal display 6 is magnified and projected by the projection lens 7 onto a screen (not shown).
In a reflection-type liquid crystal display, switching elements can be arranged under pixel electrodes. Accordingly, the pixel pitch can be shortened without reducing the size of the switching elements, and a high density can be attained without reducing the pixel aperture factor. Therefore, a reflection-type liquid crystal display can produce a projection image which is brighter and has a higher resolution than that produced by a transmission-type liquid crystal display.
In the configuration of the projection display apparatus shown in FIG. 16, when the S-polarized light 4 entering the liquid crystal display 6 is reflected therefrom without being converted into P-polarized light, and again reflected by the polarization beam splitter 3 toward the light source 1, a black display is conducted. At this time, if a portion of the S-polarized light fails to be reflected by the polarization beam splitter 3 and passes therethrough to enter the projection lens 7, the contrast of a resulting projection image is greatly impaired. In order to obtain a high-contrast projection image, therefore, the polarization beam splitter 3 must be designed so as to have a very small transmittance of S-polarized light.
In contrast, the light converted into P-polarized light by the liquid crystal display 6 passes through the polarization beam splitter 3, and is then projected on the screen so as to conduct a white display. In order to obtain a highly bright projection display, therefore, the polarization beam splitter 3 is required to have a high performance of transmitting P-polarized light.
Generally, as the polarization beam splitter 3, mainly used is a type which is proposed by S. M. MacNeille in U.S. Pat. No. 3,346,319 and in which two glass prisms are attached together to form a cube or a rectangular parallelepiped, and a multilayer optical thin film is formed at the interface of the two glass prisms. The multilayer optical thin film is structured by alternately stacking two kinds of thin layers having different refractive indices, and splits natural light into two polarized light components having polarizing planes which perpendicularly cross each other, by utilizing Brewster's angle and the interference effect of light. The material and film thickness of the multilayer film are selected so as to satisfy the Brewster's angle condition in which the transmittance of P-polarized light of a specific wavelength is 100%. When the Brewster's angle is indicated by .theta..sub.G, the refractive index of the glass prism by n.sub.G, the refractive index of low refractive index layers by n.sub.1, and the refractive index of high refractive index layers by n.sub.2, the Brewster's angle is expressed by the following expression: ##EQU2##
When the incident angle to the multilayer film face equals the Brewster's angle, the transmittance of S-polarized light can be reduced by increasing the number of layers of the multilayer film, while maintaining the transmittance of P-polarized light to be 100%.
However, the performance of a polarization beam splitter of this kind depends on the incident angle of light. When the incident angle is shifted from a reference incident angle, particularly, the transmittance of P-polarized light is greatly reduced.
Hereinafter, the reference incident angle of light indicates an angle designed so that the multi-layer film operates to effectively split the light incident on the film at the angle into two polarized light components. The incident angle of light indicates an angle at which main portion of the light is incident on the multilayer film. In other words, the incident angle is an angle between a beam axis of light and the plane of the multilayer film.
In the configuration of FIG. 16, light which is not perfectly parallel enters the polarization beam splitter 3 in most cases, and therefore the above-described incident angle dependence of the performance of the polarization beam splitter 3 causes the light efficiency to be extremely lowered.
As an example, spectral transmittance characteristics of a polarization beam splitter are shown in FIG. 17 which are obtained in the case where, in expression (1), the reference incident angle to the multilayer film face is 45.degree., the refractive index of the low refractive index layers is 1.46, the refractive index of the high refractive index layers is 2.30, and the refractive index of the prism is 1.74. The polarization beam splitter includes a multilayer film having an alternating structure consisting of 13 layers in which the first and thirteenth layers counted from the side of the coated face are low refractive index layers having an optical thickness of .lambda..sub.0 /8 (.lambda..sub.0 =730 nm, hereinafter .lambda..sub.0 indicates the main wavelength of design), and the other layers have an optical thickness of .lambda..sub.0 /4. It will be seen that there is a wavelength at which, when the incident angle of light is changed by .+-.5.degree. from the reference incident angle, the transmittance of P-polarized light is reduced by 50% or more at the maximum.
In order to prevent this problem from arising, the range of the incident angle of light can be reduced by making small the effective aperture of the optical system. When the effective aperture of the optical system is made small, however, the light efficiency is lowered, and therefore it is difficult to provide a projection image which is sufficiently bright.